As we know, strong lights are harmful to our eyes. But, in some jobs, strong lights, such as caused in electric welding, are unavoidable so that eye protection is quite indispensable.
In the earlier days, the way to protect the eyes was to wear blackcolored glasses which were inconvenient for wearers. Nowadays, though, there has been a sort of automatic liquid crystal light-shutter; it uses two photo diodes to detect respectively the direct light and the lateral light, changing them into electric signals which are compared in an electric circuit, enabling a liquid crystal display (LCD) controlled by the electric circuit to let light penetrate through or to shut it out according to the light difference.
The above-mentioned equipment based on comparing the different intensities of two lights is widely applied in a variety of devices. But it acts too slowly; when applied in detecting the light of electric welding so that strong light at the very beginning of work still more or less makes workers' eyes feel uncomfortable. Also, when a worker is working directly under the sunshine, the lateral light is so strong that it will influence the responding sensitivity, and when the object is reflective, the light reflected from the object will be erroneously detected as direct light, interrupting its detecting preciseness.
In order to improve the disadvantage of the above-mentioned equipment, the inventor has worked hard to provide this invention that is based on using a photo diode to detect directly the rapid flashing light caused by the AC power in electric welding and then makes use of an electric circuit for controlling a liquid crystal display to operate; quickly.